U.S. Pat. No. 7,738,113 discloses a wafer measurement system and apparatus for the measurement of wafer thickness, flatness and trench depth. The back surface of the wafer is used to accurately measure the backside of a trench, thus rendering the trench an effective bump that is capable of being measured on the top surface and the bottom surface through a non-contact optical instrument.
Increasing miniaturisation of semiconductor devices tends to lead to the miniaturisation of the features within the devices including features such as trenches and vias. Features such as trenches and vias have a high aspect ratio. With increasing miniaturisation, in particular, with increasing lateral shrink, the aspect ratio of these features may also increase. However, an increase in the aspect ratio further increases the difficulty of measuring the depth of the feature. Therefore, further developments in apparatus and methods are desirable in order to be able to better determine the depth of features having a high aspect ratio.